Another factor in a beginning player playing 'lefty' is that his chords will all appear different from yours. And any video will also be next to useless to watch. If the three of you are practicing together (love it!) this could be a constant frustration.

I have a musician friend who has brothers, all of whom played guitar. As the youngest in the family, he learned to play right-handed; as he said, if he hadn't he would never have gotten to play at all.

Nowadays, in his 40s, he has several left-handed instruments which are his preference.